1. Field of the Invention
This invention relates to improved electrically conductive polydiorganosiloxane compositions and to their use in ignition cable on spark ignited engines.
2. Description of the Prior Art
Electrically conductive compositions composed of materials filled with electrically conducting particles are well known in the art. Powders and fibers of metals and metal oxides as well as of carbon and graphite have been disclosed as useful in making compositions electrically conductive. The materials suggested have varied from a volatile vehicle such as alcohol to many types of resins and elastomers. Conducting compositions have been suggested for use in making resistance heating devices, electrical switches, shielding, gasketing, and electrical conductors.
In U.S. Pat. No. 3,029,808, Kegan describes a voltage detection system designed to measure voltages in the order of one millivolt. This detection system has an electrode which contacts the skin of a living animal. Kegan describes a satisfactory electrode which is composed primarily of a methyl phenyl polysiloxane gum with a filler of carbon black in an amount of 15 to 40 parts plus a vulcanizer of dicumyl peroxide.
In Canadian Pat. No. 653,350, Culver describes an ignition lead comprising a silicone rubber filled with electrically conductive carbon black or graphite and optionally a silica. The silicone rubber defined by Culver is a well-known elastomeric material based on organosiloxane polymers of the general unit formula ##EQU1## wherein R is a monovalent hydrocarbon radical or a halogenated monovalent hydrocarbon radical and n is 1.95 to 2.01. The silicone rubber can be vulcanized only by certain vulcanization agents because the presence of carbon black and graphite limits the vulcanization systems which can be used.
Culver teaches that at least 25 parts by weight conductive filler is required to give satisfactory conduction where the parts by weight are based on 100 parts by weight of siloxane polymer. Culver also teaches that a reinforcing silica filler is required to provide improved physical properties and that a filler loading of 40 parts by weight conductive carbon black or graphite and 25 parts by weight fume silica is an excellent combination of desired conductivity and good physical properties. The claims of Culver require at least 25 parts by weight conductive carbon black or graphite and at least 15 parts by weight fume silica both based on 100 parts by weight diorganosiloxane polymer. These compositions contain ditertiary butyl peroxide or dicumyl peroxide as vulcanizing agent. Culver also teaches that the use of some phenyl substituents present in the organosiloxane polymers can improve the heat stability.
The use of a conductive rubber coating in automotive ignition cable constructions is discussed in U.S. Pat. No. 3,284,751 by Barker and Shank. The conductive rubber (not further identified) dissolved in a solvent is used to impregnate a group of filaments with a conducting rubber.
Vitale, in U.S. Pat. No. 3,680,027, claims an ignition cable comprising a core and insulation, said core comprising a plurality of flexible fibers bonded together with a conductive silicone dispersion rubber.
The increased severity of the use requirements placed on ignition cables with passage of time has necessitated a search for acceptable materials o construction. The fibers used have changed from organic fibers such as linen and polyester to glass. The insulation has changed from styrene-butadiene to ethylene-propylene terpolymers or silicone rubber. In many cases, silicone rubber is also used as the jacketing material to provide the long-term heat aging requirements being placed on the cables. In addition to the higher use temperature requirements, the operating voltage of automotive ignition systems is also being increased from a nominal 17,000 volts of the past to as high as 40,000 volts at present.
The resistive conductor construction materials used in the past are no longer able to function for a sufficient length of time at the newly required temperatures and voltages. A means of evaluating ignition cable is a "High Energy Accelerated Life Test" described below. This test was originally run at room temperature with a standard ignition coil supplying the voltage. The test now requires use of new, higher voltage coils with the test specimens being tested at 260.degree. C.
Previous ignition cables have used cores produced by using conductive acrylic dispersions, conductive silicone dispersions, and conductive silicone emulsions. None of these materials meet the present requirements. The improved compositions of this invention allow the manufacture of ignition cable core which will meet the current requirements.
The compositions of the prior art used in the manufacture of resistive core as the conductive member in automotive ignition cables were usually hydrocarbon solvent dispersions or water emulsions. A great deal of energy is used to dry and cure such a coating. The preferred compositions of the present invention do not require a solvent so the energy used to cure the coating is a small fraction of that previously used.